That website has a calculator to help put a number to the recoil of a gun based on weight and load used, it won't tell you anything about how the gas system will affect recoil though. For the SA-28, are you looking at the tactical turkey model? The pistol grip on that should help reduce felt recoil as well, along with just being handy. I haven't found the answer to this yet, but with a synthetic stock, it should have some cavities you can add weights to to up the mass of the gun and reduce recoil some. Running a full mag on the semi will help out and add a couple ounces of weight, here in MO you can only have a three shell capacity, but I think you could use something heavier than the plastic or wooden dowels the factory sends to restrict capacity, maybe a length of PVC with some lead shot epoxied in it